The present invention relates to a balance adjustment method of an end mill in an apparatus for cutting with the rotating end mill, such as a milling machine and a machining center, in which unbalance of the end mill having asymmetric cutting parts is adjusted to prevent movement dispersion and vibrations of cutting edges so as to increase the cutting efficiency and to enhance the machining accuracy and the surface roughness.
The machining accuracy required for a machining apparatus has been further enhanced in response to the progress of numerical control and the establishment of systems for correcting control of errors of the apparatus, temperature control and torque control.
However, due to an increase in the centrifugal force in accordance with a speed increase of a spindle (the rotational speed: 20,000 to 100,000 r.p.m.) in recent years, mass unbalance of an end mill having asymmetric cutting parts causes many phenomena conspicuously. For example, the unbalance induces vibration of a machining apparatus, deteriorates the roughness of a machined surface, and further, shortens the lifetime of the end mill by partial contacts of cutting edges, chipping generated by vibrations or the like.
As disclosed in "All about the End Mill" (published by Taiga Publishing Company on Jun. 1, 1988, p. 146), conventional configurations of asymmetric cutting parts are observed in two-, three-, four- and six-flute end mills. However, unbalance is generated in three types of these end mills, i.e., two-, three- and four-flute end mills.
An end mill having asymmetric cutting parts is designed in such a manner that one of a plurality of end cutting edges is shaped to extend over the center of revolution in order to improve the cutting efficiency of the central portion of revolution and to facilitate boring of a workpiece. This end cutting edge is called a primary cutting edge and the others are called secondary cutting edges in some cases. Generally, however, an end mill having such a configuration is called an end mill having asymmetric cutting parts.
This type of an end mill having asymmetric cutting parts is characterized in that the single end mill can perform both boring and machining of an inner peripheral surface, as observed in the case of grooving of a key groove. The number of cutting edges is selected in accordance with a material quality and a machining purpose. Usually, an end mill having a small number of cutting edges is used for rough machining, and the number of cutting edges is increased as the finishing accuracy is raised.
By use of a two-flute end mill having asymmetric cutting parts as an example, problems will be discussed with reference to FIGS. 2A and 2B. In this example, the longer end cutting edge is called an end cutting edge A and the shorter end cutting edge is called an end cutting edge B for the sake of convenience.
Of the two end cutting edges, the end cutting edge A 1 is shaped to extend over the center of revolution 6. Therefore, in order to obtain a tooth face 5 in this end of cutting part, an end gash part A 3 is lightened by a larger degree than an end gash part B 4.
When such an end mill having asymmetric cutting parts is rotated, the centrifugal force of the end gash part B becomes larger. As a result, the end mill is rotated in an unbalanced manner in this direction, and a degree of the unbalanced rotation is increased as the rotational speed is higher.
Especially when this end mill is used in a recent high-speed machining apparatus or the like, the degree of unbalanced rotation of the end mill is increased in proportion to the rotational speed. In consequence, even if a plurality of cutting edges are employed, cutting tends to be actually conducted by only one of them. In this case, the lifetime of the tool will be extremely shortened, and also, the machining accuracy and the surface roughness will be largely affected.
Moreover, whirling in the rotation owing to unbalance of the end mill causes a load more than necessary to be exerted on a spindle. Furthermore, vibration generated by the unbalanced rotation is apt to increase the roughness of the machined surface and the occurrence of chipping of cutting edges of the end mill.